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Parallel Power Transformers

11/27/2005 2:14 PM
Parallel Power Transformers
Not tube related but a general theory/practice type of question:  
I have two toroidal transformers, each with 117VAC primary and 50VAC-0-50VAC secondary at 7.5A. If my circuit needs 10A @ +/-70VDC, can I put them in parallel to increase the current capability? Secondaries should be matched/etc. to balance out the current draw, but ideally, do I put them parallel (a) before the rectifiers (two bridge rectifiers) or (b) after the rectifiers (so I'd have to use four bridge rectifiers)?  
Any pointers appreciated.
11/27/2005 3:40 PM
Ray Ivers
In theory, two identical transformers can be paralleled with no problems at all. In practice... I don't know, I've never had the balls to parallel two PT secondaries. ;) I've parallel- and series-connected damn near everything else, though. I much prefer using series connections for any transformer windings (or the ideal iron for the job, but that's not always feasible).  
My Audio Gadgets program tells me you can expect 65VDC @ 6.2A from each of two FWCT circuits, one for each PT secondary; this is the way I would do it with those PT's. With the paralleled connection you suggested, the program indicates +/- 65VDC @ 8.4A. My gut tells me that if you have problems with the parallel connection using toroids, they will be serious, instantaneous problems.  
Hmmm, something like 1,500VA goin' on here; I guess you're building a single-12AX7 preamp? :D  
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11/27/2005 4:35 PM

Thanks for the insights, Ray. Googling around I found a few links that indicated it *should* be OK provided the secondaries are putting out exactly the same thing, which isn't always the case in practice. There was some lamination and bifilar stuff which was over my head but the gist of what I understood was OK. Just wondering if anyone has done this before.  
I need the bipolar supply for a (gasp!) solid state amp, the details of which are irrelevant for here :) I got a screaming deal on the iron ($10 each!) thinking I could wire the secondaries of a single transformer in parallel and wire separate + and - bridge rectifiers after that, but it turns out there are only two connections w/center tap (three total) on the secondaries so that's a no go.  
HMM, time to visit the library methinks!
11/27/2005 6:11 PM

If they are the same make and model you can probably get away with it fine. many makers sell dual secondary PTs with the ratings given for series or parallel use. it is no different from a dual primary PT which you can wire either series or parallel for 120 or 240 volt operation.
11/27/2005 7:21 PM
Don Symes

I'm not sure I can ask this coherently, but it's the seed of an idea ...  
If you:  
Tie the secondaries' CTs together and ground them, and,  
Give each outer secondary tap a 'positive' and 'negative' diode which respectively feed the positive and negative filter banks, and,  
Wire the primaries in reverse phase,  
do you get the paralleling effect for current, but minimize the risks?  
Key words: Inrush Current Limiters (one for each core).  
/ oh - and stacking the toroids can put _really_ large, opposing mechanical forces on both trannies such that they try to separate from each other at high speed, alternating with trying to slam together, at 120Hz or so.
11/27/2005 6:13 PM

Keep in mind you will have two of them with their big power up yank. ANd I wouldn't stack them if I were you.
11/27/2005 6:26 PM
Ray Ivers
What happens when you stack toroids?  
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